In water treatment systems dissolved and suspended solids are separated from the liquid so that the liquid can be processed for re-introduction into the water supply or discharged harmlessly into reservoir or estuary such as a river or stream.
Typically a centrifuge comprising a bowl whose interior rotatably supports a scroll conveyor is used at an early stage in water treatment to assist in the separation of the solids.
As is well known, the mixture of raw water and solids, often called a "feed slurry," enters the centrifuge at one end of the bowl. The solids are separated by the interaction of gravity, the speeds of rotation of the centrifuge bowl and the scroll as well as their relative speeds of rotation.
Additionally a catalyst is often used to assist in the separation of the solids. Typically the catalyst is a suitable electrically charged polymer. The polymer charge may be positive or negative in accordance with the nature of the separation to be achieved as will be more fully explained. Suitable polymer is available from Cytec, Industries, Inc. (formerly known as American Cyanamid Corporation) in West Paterson, N.J. under the brand name SUPERFLOC C-457 which is a cationic polyacrylamide family.
The solids are collected and can be used for a variety of purposes such as fertilizers, landfills, fuels for incinerators and the like.
It is recognized that the solids are best handled when their consistency is at a predetermined level. Preferably, the solids should be dry since this makes them easier to handle. If they are too wet, they become heavy as a result of the water they contain and are costly to handle and ship.
Thus, it is well known in the industry to try to control the density of the solids exiting the centrifuge by regulating relative speeds of the scroll and the centrifuge bowl.
However, this technique is limited by the density of the feed slurry entering the centrifuge.
Further refinements in controlling the density of the solids exiting the centrifuge include the recirculation of the solids to increase their concentration in the centrifuge so that the density of the solids finally exiting the centrifuge can be controlled.
While this means for controlling the density of the solids exiting the separator has been satisfactory for the most part, it suffers from the following deficiencies.
Since the density of the exiting solids is substantially proportional to the density of the feed slurry, a very dense feed slurry will create solids that tend to reduce the flow of material through the centrifuge or even block it entirely. Therefore, a very dense slurry will cause the rate at which the centrifuge unloads to increase and remain high until a feed slurry having a lighter density is detected. This causes an uneven product since the exiting solids will have alternately dry and wet portions in accordance with the density of the feed slurry. The increased wetness of the solids increases the cost of disposal since the wet solids cost more to transport than dry solids due to their weight and further, the increased moisture makes them more difficult to incinerate.